Well, like Koir said, most of the time when people here meet up for things like cookouts, football games, and even movies, it's better to bring food and drinks with you. I guess this is often seen as a way to relieve the host from having to supply everyone with food on their own. It's really just a social convention that most people will choose to follow though. Therefore, it's not entirely necessary to bring anything at all, especially if you're going by yourself or don't plan on staying too long.
In my experience, the only real gift giving is done around the more obvious, major events (Christmas, Birthdays, Valentines Day...) although it's typical for one to receive gifts during other important stages of their life as well (graduation, buying their first home, getting married, having kids, getting promoted, retiring, etc...).
I hope this has been helpful.
